Transport refrigeration systems or units, such as associated with straight trucks, tractor trailer combinations, and refrigerated containers (reefers), include those which have a dedicated prime mover for the refrigerant compressor, and those in which the compressor is driven by the engine of the associated vehicle. The latter are usually associated with small straight trucks and refrigerated delivery vans.
Transport refrigeration units normally condition a fresh or a frozen load in an associated cargo space to a predetermined set point temperature selected by a driver or attendant. The transport refrigeration unit automatically maintains the cargo space within a predetermined narrow temperature range adjacent to the selected set point temperature via cooling and heating cycles, as required. During a cooling cycle a refrigerant compressor directs refrigerant through a closed refrigerant flow circuit which includes a condenser and an evaporator. During a heating cycle to hold set point temperature, as well as for defrosting purposes, it is common to direct the refrigerant from the compressor directly to the evaporator, by-passing the condenser. Thus, cooling cycles and heating cycles may properly be termed refrigeration cycles.
With a dedicated prime mover, the prime mover is under the control of the transport refrigeration system, and may be started to initiate a predetermined refrigeration cycle, ie., a cooling cycle or a heating cycle, and stopped, as dictated by the requirements of the served cargo space. This start-stop operating mode, as opposed to a continuous operating mode, conserves fuel, when the prime mover is an internal combustion engine, and it conserves electricity, when the prime mover is an electric motor.
When the refrigeration system prime mover is an engine which also drives the associated truck or van, the prime mover is completely under control of the driver. Some transport refrigeration units in which the compressor is driven by the vehicle engine have a stand-by electric motor for driving the compressor when the vehicle is accessible to a source of electric potential. This standby arrangement is of no use, however, while the vehicle is making deliveries. Thus, when the driver leaves the vehicle, such as to make a delivery, lunch break, overnight away from the base terminal, and the like, the engine is either left running, which is wasteful of fuel, or it is turned off, which may endanger the load if the driver is delayed in returning to the vehicle. On a warm day, a load of fresh flowers in a delivery van, for example, may be quickly damaged if the driver turns the engine off upon leaving the van, and the driver is delayed in returning.
Thus, it would be desirable, and it is an object of the invention, to safely place an engine, which drives both the wheels of a vehicle and a refrigerant compressor, under control of the associated transport refrigeration system in predetermined circumstances, to maintain the cargo space of the vehicle at set point temperature during the absence of the driver, without being wasteful of engine fuel.
It would also be desirable, and it is a further object of the invention, to control the vehicle engine RPM, while operating under control of the transport refrigeration unit, to match engine RPM to the requirements of the served cargo space at any instant. This precludes the necessity of setting the engine idle RPM to a maximum value selected to handle the most extreme cargo demand conditions which may be encountered, again conserving engine fuel.